1. Field of the Invention
The present invention generally relates to humidity control systems employed to maintain a desired relative humidity level during a particular process. More specifically, this invention relates to a humidity and temperature control system adapted for maintaining a predetermined relative humidity and temperature level within a paint spray booth during the painting of automobile bodies.
2. Description of the Prior Art
Until recently, solvent-based paints have been the preferred paint for applications in the automotive industry, such as the spray painting of automobile bodies, appliances and fiberglass finishing. Such spray painting is conducted within an enclosed spray booth to prevent paint overspray solids and solvents from contaminating other areas of a manufacturing facility, while also preventing entry of air-borne particles. To prevent accumulation of paint overspray within the paint spray both, outside air is forced through the enclosure and vented to atmosphere. The ventilation systems must have the capacity to move sufficiently large volumes of air to keep the accumulation of paint overspray to a minimum.
In response to environmental concerns, both manufacturers and federal regulators have begun to emphasize the use of water-based paints. The conversion to water-based paints has been mandated to some degree by federal regulations which have imposed restrictions on the emission of volatile organic compounds, such as hydrocarbons, which are present in solvent-based paints. As a result, water-based paints have become even more attractive for use within the automotive industry.
However, a disadvantage with water-based paints is their limited ability to produce a satisfactory finish unless the proper conditions are maintained within the paint spray booth. In large part, this drawback is due to the tendency for the water component of a water-based paint to evaporate before the water-borne paint particles come in contact with the surface to be covered. Accordingly, the paint particles reach the surface in a substantially dry form, making it practically impossible to produce an acceptable surface finish. As a consequence, the ventilation air which is forced through the spray paint booth must be humidified sufficiently to prevent the water component of the water-based paint from evaporating prior to contacting the desired surface.
Various approaches have been suggested to provide a suitable humidity control system, many of which include some form of air conditioner or heat exchanger having both a heating and cooling capability to regulate the temperature and relative humidity of the ventilation air, such as that disclosed in U.S. Pat. No. 4,616,594 to Itho. In contrast, U.S. Pat. No. 3,979,535 to Govindan teaches the direct spraying of water or steam into the ventilation air, though how the control system taught by Govindan is able to handle the rapid changes in temperature and relative humidity due to the use of the direct spray method is not disclosed. Primarily, Govindan is directed to the problem of formulating a water-based paint which is suitably matched to the particular specific relative humidity condition provided in the paint spray booth.
More explicit examples of solutions which address how the temperature and relative humidity are controlled are illustrated in U.S. Pat. Nos. 4,173,924 and 4,367,787 to Bradshaw. The former teaches the use of a bank of water spray nozzles which are directed at a first heat exchanger from which the water evaporates to saturate the air passing through the heat exchanger. A single pair of sensors are employed, one being a wet-bulb temperature sensor located immediately downstream of the first heat exchanger and the second being a dry-bulb temperature sensor located downstream from a reheating heat exchanger which reheats the air after it leaves the first heat exchanger.
The later Bradshaw Patent also uses water spray nozzles which spray water onto a heat exchanger. But in contrast, there is disclosed, in combination with a bypass duct, the use of a pair of sensors which are located both upstream and downstream of the heat exchanger. The feedback from the sensors is provided to a controller to suitably adjust the amount of air bypassed around the temperature and humidity devices through the bypass duct. The desired relative humidity of the air is thereby controlled by the amount of air bypassed through the bypass duct.
The above systems generally teach a complicated series of humidifying-cooling/reheating steps, including heat exchangers for recapturing heat downstream of the paint spray both, in order to achieve the desired temperature and relative humidity levels. Such an approach is expensive, requiring a vast array of valves, pipes and electrically-powered heat exchangers which are rather complicated and expensive to maintain. Moreover, such systems require a rather complicated control system to synchronize all of the devices.
Further complicating a control system used under these circumstances is its ability to bring the system up to, and thereafter maintained at, the desired conditions while a large volume of air is rapidly traveling through the air ducts and the paint spray booth. The above-described approaches avoid one source of difficulty by using air conditioners and heat exchangers which react relatively slowly to commands from the control system. This relative insensitivity avoids any rapid changes in the relative humidity level of the ventilation air, which would be sensed almost immediately by the humidity sensor and then fed back to the controller. Sudden changes in humidity would require a correspondingly quick modification in the controller's signal to the humidifying device which, coupled with subsequent rapid changes in relative humidity in response to the modified signal, would cause the system to become unstable and cycle uncontrollably about the desired relative humidity level. Accordingly, the approach taught by the prior art is stable, but comes at the cost of rather high expenses associated with the purchase and maintenance of the air conditioner and heat exchanger systems.
Therefore, what is needed is a humidity control system for a paint spray booth which is simplified and less expensive, and yet employs reliable control features which are stable and can maintain the desired temperature and relative humidity conditions in the paint spray booth.